Recently, as a communication scheme for a rapidly developing mobile communication system, for example, PHS (Personal Handy phone System), a TDMA scheme in which 1 frame (5 ms) consisting of respective 4 slots (1 slot: 625 μs) for transmission and reception is regarded as a base unit has been adopted. Such a communication scheme for PHS is standardized as the “second generation cordless communication system,” for example.
A signal of 1 frame is divided into 8 slots, that is, first 4 slots serve for reception, while following 4 slots serve for transmission, for example.
Each slot consists of 120 symbols. For example, in a signal of 1 frame, assuming that one reception slot and one transmission slot form one pair, three pairs of slots are allocated as traffic channels for three users, and remaining one pair of slots is allocated as a control channel, respectively.
In the PHS system, in a control procedure for establishing synchronization, a link channel is initially established by the control channel, followed by a processing for measuring an interference wave (an undesired wave: U wave). In addition, after a processing for setting communication condition by the allocated channel, speech communication is started. Such a procedure is disclosed in detail in Personal Handy Phone System RCR Standard RCR STD-28 (published by Association of Radio Industries and Businesses), which is a standard of PHS.
FIG. 19 shows a flow in such a communication sequence in PHS. In the following, brief description thereof will be provided with reference to FIG. 19.
First, a C channel (control channel: CCH) is used to transmit a link channel establishment request signal (LCH establishment request signal) from a PHS terminal to a base station. A PHS base station detects an empty channel (empty traffic channel: empty T channel) (carrier sensing), and uses the C channel to transmit a link channel allocation signal (LCH allocation signal) designating an empty T channel to the PHS terminal.
In the PHS terminal, whether or not an interference wave signal having a power larger than a prescribed level is received is measured in the designated T channel (U wave measurement) based on link channel information received from the PHS base station. When the interference wave signal with a power larger than a prescribed level is not detected, that is, when other PHS base station does not use the designated T channel, the PHS terminal uses the designated T channel to transmit a synchronous burst signal to the base station. Meanwhile, the base station sends back a synchronous burst signal to the terminal. Synchronization is thus established.
On the other hand, when an interference wave signal having a power larger than a prescribed level is detected in the designated T channel, that is, when the T channel is being used by other PHS base station, the PHS terminal repeats the control procedure from the link channel establishment request signal.
In this manner, in the PHS system, a traffic channel between a terminal and a base station is connected, using a channel where the interference wave is weak and excellent communication performance is attained.
In the PHS, a PDMA (Path Division Multiple Access) scheme has been implemented, in which, in order to enhance an efficiency in utilizing a frequency of a radio wave, mobile radio terminal units (terminals) of a plurality of users establish spatial multiple connection to a radio base station (base station) through a plurality of paths formed by spatially dividing an identical time slot of an identical frequency.
The PDMA scheme adopts an adaptive array technique, for example. In an adaptive array processing, based on a reception signal from a terminal, a weight vector consisting of reception coefficients (weights) for respective antennas in the base station is calculated for adaptive control, and a signal from a desired terminal is accurately extracted.
With such an adaptive array processing, an uplink signal from the antenna of each user terminal is received by the array antenna of the base station, and then separated and extracted with reception directivity. A downlink signal from the base station to the terminal is transmitted from the array antenna with transmission directivity to the antenna of the terminal.
Such an adaptive array processing is a well-known technique, and described in detail, for example, in Nobuyoshi Kikuma, “Adaptive Signal Processing by Array Antenna”, Kagaku Gijutsu Shuppan, pp. 35-49, “Chapter 3: MMSE Adaptive Array” published on Nov. 25, 1998. Therefore, description of its operation principle will not be provided.
FIG. 20A is a conceptual view schematically illustrating an example in which one terminal 2 with a single antenna is connected to a PDMA base station 1 via one of a plurality of paths formed by space division in a mobile communication system (PHS) adopting the PDMA scheme.
More specifically, PDMA base station 1 receives an uplink signal from one antenna 2a of terminal 2 with an array antenna 1a, and the signal is separated and extracted with reception directivity through the above-described adaptive array processing. On the other hand, array antenna 1a of PDMA base station 1 transmits a downlink signal with transmission directivity to one antenna 2a of terminal 2. Terminal 2 receives the downlink signal with its antenna 2a without adaptive array processing.
FIG. 20B is a timing chart schematically showing a manner of channel allocation in this example. In the example of FIG. 20B, users 1 to 4 establish time-division multiplexed to respective time slots obtained by division in a direction of time axis at an identical frequency. Here, one user is allocated to each slot via one path in a spatial direction.
Identification of a desired signal out of signals received in the PDMA scheme is performed in the following manner. A radio wave signal transmitted/received between a terminal such as a mobile phone and a base station is transmitted in what is called a frame configuration including a plurality of frames. For example, each frame includes a total of 8 slots, that is, 4 slots for uplink communication and 4 slots for downlink communication. Broadly speaking, the slot signal is constituted of a preamble consisting of a signal sequence already known to a reception side, and data (such as voice) consisting of a signal sequence unknown to the reception side.
The signal sequence in the preamble includes a signal train (reference signal: unique word signal, for example) of information for discerning whether or not a sender is a desired party for the reception side to establish communication. For example, an adaptive array radio base station performs weight vector control (determines weight coefficient) so as to extract a signal that seems to include a signal sequence corresponding to a desired party, based on comparison of the received signal sequence with the unique word signal taken out from a memory.
In addition, each frame is assumed to have a configuration in which a unique word signal (reference signal) section described above is included and cyclic redundancy check (CRC) is enabled.
In contrast, an MIMO (Multi Input Multi Output) scheme has been proposed, in which multiplex communication is established between one terminal having a plurality of antennas and a PDMA base station via a plurality of spatial paths of an identical frequency and an identical time slot.
Communication technologies for such MEMO scheme are described in detail, for example, in Nishimura et al., “SDMA Downlink Beamforming for a MIMO Channel,” Technical Report of MICE, A-P2001-116, RCS2001-155, pp. 23-30, October 2001, and in Tomisato et al., “Radio Signal Processing for Mobile MIMO Signal Transmission,” Technical Report of MICE, A-P2001-97, RCS2001-136, pp. 43-48, October 2001.
FIG. 21 is a conceptual view schematically illustrating an example in which one terminal PS1 with two antennas establishes spatial multiple connection to a PDMA base station CS1 via a plurality of paths (e.g. two paths) PTH1, PTH2 formed by space division in the mobile communication system (PHS) adapted to such MIMO scheme.
More specifically, PDMA base station CS1 receives uplink signals from respective two antennas 12a, 12b of terminal CS1 with an array antenna 11a, and the signals are separated and extracted with reception directivity through the above-described adaptive array processing.
On the other hand, array antenna 11a of PDMA base station CS1 transmits downlink signals with transmission directivity to respective two antennas 12a, 12b of terminal PS1. Terminal PS1 receives corresponding downlink signals with its respective antennas without adaptive array processing.
FIG. 22 is a timing chart schematically showing a manner of channel allocation in this example. In the example of FIG. 22, users 1 to 4 are time-division multiplexed to respective time slots divided in a direction of time axis at an identical frequency. An identical user is allocated in a manner of multiple connection to each slot via two paths in a spatial direction.
For example, noting a first time slot in FIG. 22, user 1 is allocated to all channels via two spatial paths. Then, a signal of user 1 is divided and transmitted between the terminal and the base station via two paths in the identical slot, and the divided signals are reconfigured on the reception side. Two-paths-for-one-user scheme as shown in FIG. 22 can double a communication speed, as compared with one-path-for-one-user scheme in FIG. 20B.
Here, some of the plurality of spatial paths in the identical slot in the PDMA scheme may be used to establish communication in multiple-paths-for-one-user scheme as shown in FIG. 21. Concurrently, remaining paths may be used to establish communication in one-path-for-one-user scheme as shown in FIGS. 20A and 20B.
A specific method of transmission/reception of a signal in the MIMO scheme as shown in FIG. 21 is disclosed in detail in Japanese Patent Laying-Open No. 11-32030, for example.
In the MIMO scheme as shown in FIG. 21, the terminal side prepares antennas in the number corresponding to the number of paths to be set, so as to establish communication.
If a failure occurs on a propagation path, however, there is no degree of freedom in that path allowing for avoiding such a failure. Eventually, disconnection of the path has been likely.
In this manner, though improvement in the communication speed can be expected with the conventional MIMO scheme in an environment where a condition for communication is excellent, it is difficult in some cases to achieve a stable communication speed.
Accordingly, an object of the present invention is to provide a radio apparatus capable of adaptive modification in connection of a plurality of paths between a terminal and a base station in accordance with a communication condition in a mobile communication system where communication is established with a multiple-paths-for-one-user scheme such as the MIMO scheme, as well as a method and a program for controlling a spatial path.